This invention relates to impedance matching networks and more particularly to an impedance matching network used for coupling a microwave transmission line to a field effect transistor (FET).
As is known in the prior art, the application of a field effect transistor amplifier for a multioctive bandwidth application presents severe impedance matching problems when the amplifier is used with a microwave transmission line becuase of the reactive characteristics of the input impedance of the FET. If the impedance matching over a broadband of frequencies is attempted with purely reactive elements to compensate for the impedance of the FET, as by a plurality of series and parallel connected reactive elements, the gain of the amplifier will not generally be uniform over the entire operating bandwidth. One technique for impedance matching using a pure passive element is suggested in an article entitled "Device-Circuit Considerations in the Design of Broadband MESFET Amplifiers, by Ganesh R. Basawapatna, published in conference record of the Eleventh Annual Asilomar Conference On Circuits, Systems and Computers, papers presented Nov. 7-9, 1977. The technique suggested in this article consists of placing a resistor between the gate and source electrodes of an FET. While this technique achieves a useful degree of impedance matching especially when the resistance of the resistor is relatively low, in the order of 200 ohms, nevertheless a severe loss of gain may result from the use of a pure resistive load.
Further, manufacturing difficulties may be encountered in fabricating reactive elements, such as an inductor as a part of the compensating network. In particular, an inductor for use in microwave frequency circuits may generally be realized as a coiled inductor, a one mil etched line on a semi-insulating substrate, or as a bond wire. Generally, a preformed coiled inductor is used to realize a coiled inductor in a microwave frequency circuit. A preformed coiled inductor may be unsatisfactory for some applications because the inductor may receive deformation forces due to handling and bonding to the circuit elements. The changes in the electrical characteristics of the inductor which may result include an increase in the tolerance of the inductance, decrease in the degree of reproducibility of an inductor having a desired inductance value, and an increase in degree of unpredictability of parasitic capacitance and inductance resulting from a nonreproducible placement of the inductor in relation to other circuit elements.